There is a growing demand for transportation fuels made from renewable feedstocks. These renewable fuels displace fossil fuels resulting in a reduction of greenhouse gas emissions, along with other benefits.
In North America fuel ethanol is the major transportation fuel. The feedstock for fuel ethanol in North America is primarily corn. Corn contains starch which is hydrolysed to glucose and then fermented to ethanol. In other countries, such as Brazil, fuel ethanol is made by fermenting the sugar in sugar cane. It is advantageous to have an additional source of sugars like glucose to make additional Biofuels.
At the other end of the spectrum of difficulty is cellulose. Cellulose is one of the most abundant organic materials on earth. It is present in many forms of biomass, including agricultural residues like corn Stover and corncobs, woody residues and other plant materials. Cellulose is a polymer of glucose, as is starch.
This invention is specifically targeting the highest lignin content materials to produce pure reactive cellulose of value on its own or as a feedstock that is easily hydrolysed to glucose and subsequently fermented to valuable product such as biofuels. Purified cellulose components are valuable for many purposes. Specifically, when purified, it may be more easily hydrolysed to glucose, which in turn may be more easily fermented to ethanol than in previous processes.
Lignocellulosic biomass is composed of three primary polymers that make up plant cell walls: Cellulose, hemicellulose, and lignin. Cellulose fibers are locked into a rigid structure of hemicellulose and lignin. Lignin and hemicelluloses form chemically linked complexes that bind water soluble hemicelluloses into a three dimensional array, cemented together by lignin. Lignin covers the cellulose microfibrils and protects them from enzymatic and chemical degradation. These polymers provide plant cell walls with strength and resistance to degradation, which makes lignocellulosic biomass a challenge to use as substrate for biofuel production.
Among the potential lignocellulosic feedstocks there is a range of lignin contents. Corncobs have a low lignin content (6%-8%), while woody crops have a medium lignin content of 10%-15%. Wood residues have even higher lignin content (20% to 30%)
Cellulose or β-1-4-glucan is a linear polysaccharide polymer of glucose made of cellobiose units. The cellulose chains are packed by hydrogen bonds in microfibrils. These fibrils are attached to each other by hemicelluloses, amorphous polymers of different sugars and covered by lignin. Hemicellulose is a physical barrier which surrounds the cellulose fibers and protects cellulose against degradation. Lignin is a very complex molecule constructed of phenylpropane units linked in a three dimensional structure which is particularly difficult to biodegrade. Lignin is the most recalcitrant component of the plant cell wall. There are chemical bonds between lignin, hemicellulose and cellulose polymers. Thus, it is desirable to use a lignocellulosic feedstock which is low in hemicellulose and lignin. There is evidence that the higher the proportion of lignin, the higher the resistance to chemical and biological hydrolysis. Pretreatment methods for the production of fermentable sugars from Miscanthus showed the existence of an inverse relationship between lignin content and the efficiency of enzymatic hydrolysis of sugars based polymers. Lignocellulosic microfibrils are associated in the form of macrofibrils. This complicated structure and the presence of lignin provide plant cell walls with strength and resistance to degradation, which also makes these materials a challenge to use as substrates for biofuel and bioproduct production. Thus, proper preparation and pretreatment is necessary to produce cellulose that is relatively pure and reacts well with catalyst such as enzymes.
The best method and conditions of pretreatment will vary and depend greatly on the type of lignocellulosic material used. Cellulose-lignin ratio is the main factor. Other parameters to consider are cellulose accessible surface area, degree of polymerization, crystallinity and degree of acetylation of hemicelluloses. An effective pretreatment should meet the following requirements: (a) production of pure reactive cellulosic fiber e.g. susceptible to enzymatic hydrolysis, (b) avoiding destruction of cellulose and hemicelluloses, and (c) avoiding formation of possible inhibitors for hydrolytic enzymes and fermenting microorganisms.
Several methods have been investigated for pretreatment of lignocellulosic materials to produce reactive cellulose. These methods are classified into physical pretreatments, biological pretreatments and physicochemical pretreatments. Physical and biological methods alone are not sufficient. Pretreatments that combine both chemical and physical processes are referred to as physicochemical processes. These methods are among the most effective and include the most promising processes for industrial applications. Lignin removal and hemicellulose hydrolysis are often nearly complete. Increase in cellulose surface area, decrease in cellulose degrees of polymerization and crystallinity greatly increase overall cellulose reactivity. Treatment rates are usually rapid. The steam explosion process is well documented. Batch and continuous processes were tested at laboratory and pilot scale by several research groups and companies. In steam explosion pretreatment, high pressure and hence high temperatures are used i.e. 160° C. to 260° C. for 1 min to 20 min. The pressure is suddenly reduced, which explosive decompression leads to an explosive decomposition of the materials, leading to defibrization of the lignocellulosic fibers.
Steam explosion pretreatment was successfully applied on a wide range of lignocellulosic biomasses with or without chemical addition. Acetic acid, dilute sulfuric acid, or sulfur dioxide are the most commonly used chemicals. In the autohydrolysis process, no acid is added as the biomass has a hemicellulose that is high in acetyl groups that are released to form acetic acid during the steaming process. The degree of acetylation of hemicelluloses varies among different sources of biomass. The pretreatment is not very effective in dissolving lignin, but it does disrupt the lignin structure and increases the cellulose's susceptibility to enzymatic hydrolysis.
The use of liquid ammonia instead of dilute acid effectively reduces the lignin fraction of the lignocellulosic materials. However, during ammonia fiber explosion pretreatment (AFEX) a part of the phenolic fragments of lignin and other cell wall extractives remain on the cellulosic surface. AFEX pretreatment does not significantly solubilize hemicellulose if compared to dilute-acid pretreatment. Consequently, hemicellulose and cellulose fractions remain intact and cannot be separated in solid and liquid streams. Furthermore, ammonia must be recycled after the pretreatment in order to reduce cost and protect the environment.
In the Organosolv process, lignocellulose is mixed with a mixture of organic solvents and water and heated to dissolve the lignin and part of the hemicellulose, leaving reactive cellulose in the solid phase. A variety of organic solvents such as alcohols, esters, ketones, glycols, organic acids, phenols, and ethers have been used. For economic reasons, the use of low-molecular-weight alcohols such as ethanol and methanol has been favored. A drawback of the Organolsolv process is the presence of hemicellulose with the lignin. An extensive overview of prior art organosolv processes is given in “Organosolv pulping”—A review and distillation study related to peroxyacid pulping”.
In the process patented by Paszner and Chang, lignocellulosic biomass is saccharified to convert pentosans and hexosans to sugars by cooking under pressure at from 180° C. to 220° C. with acetone water solvent mixture carrying from 0.05 to 0.25% by weight of acid. Whole woody material is nearly dissolved by the process yielding mixed pentoses and hexoses. Hence, delignified pulp is hydrolyzed to glucose monomers that have to be recovered from the liquor.
The Alcell pulping process and further process developments have been applied with success on woody biomass. The problem with these processes is that they result in combined hemicellulose and lignin streams i.e. black liquor that is hard to separate afterwards. Lignin is precipitated from a black liquor produced by pulping wood at high temperatures and pressures with an aqueous lower aliphatic alcohol solvent i.e. lignin is precipitated by diluting the black liquor with water and an acid to form a solution with a pH of less than 3 and an alcohol content of less than 30%.
Pretreatment of lignocellulosic biomass is projected to be the single, most expensive processing step, representing about 20% of the total cost (65). In addition, the pretreatment type and conditions will have an impact on all other major operations in the overall conversion process from choice of feedstock through to size reduction, hydrolysis, and fermentation as well as on to product recovery, residue processing, and co-product potential. A number of different pretreatments involving biological, chemical, physical, and thermal approaches have been investigated over the years, but only those that employ chemicals currently offer the high yields and low costs vital to economic success. Among the most promising are pretreatments using a combination of dilute acid- or sulfur dioxide-catalyzed steam explosion and low molecular weight alcohols.